The tariff-schedule chapter where trade-remedy duties like Section 301 and 232 are actually implemented.
Chapter 99 of the Harmonized Tariff Schedule is a special "temporary modifications" chapter that layers additional duties on top of a good's normal classification elsewhere in the schedule (Chapters 1-97). A product doesn't stop being classified under its regular chapter when a Section 301 or Section 232 duty applies — instead, a second, Chapter 99 heading (commonly seen as a "9903.xx.xx" code) is reported alongside the primary classification on the entry, and that heading is what actually carries the additional duty rate. Reading the correct Chapter 99 heading, not just the base HS code, is what determines whether — and how much — trade-remedy duty is owed on a given entry.
HTSUS General Note 3 and Chapter 99, implemented per proclamation/Federal Register notice for each program.
For a specific entry's rate, the free Tariff Calculator gives you the stacked, cited answer.